The present invention is directed generally to a process for making synthesis gas from which streams of carbon monoxide and methanol can be obtained in approximately stoichiometric proportions suitable for the manufacture of acetic acid, and more particularly to the retrofit of a methanol plant to divert a portion of the syngas from the existing methanol synthesis loop to a carbon monoxide separator and to react the methanol from the methanol synthesis loop with the carbon monoxide from the separator in approximately stoichiometric proportions to directly or indirectly make acetic acid.
The manufacture of acetic acid from carbon monoxide and methanol using a carbonylation catalyst is well known in the art. Representative references disclosing this and similar processes include U.S. Pat. No. 1,961,736 to Carlin et al (Tennessee Products); U.S. Pat. No. 3,769,329 to Paulik et al (Monsanto); U.S. Pat. No. 5,155,261 to Marston et al (Reilly Industries); U.S. Pat. No. 5,672,743 to Garland et al (PB Chemicals); U.S. Pat. No. 5,728,871 to Joensen et al (Haldor Topsoe); U.S. Pat. No. 5,773,642 289 to Denis et al (Acetex Chimie); U.S. Pat. No. 5,817,869 to Hinnenkamp et al (Quantum Chemical Corporation); U.S. Pat. No. 5,877,347 and 5,877,348 to Ditzel et al (BP Chemicals); U.S. Pat. No. 5,883,289 to Denis et al (Acetex Chimie); and U.S. Pat. No. 5,883,295 to Sunley et al (BP Chemicals), each of which is hereby incorporated herein by reference.
The primary raw materials for acetic acid manufacture are, of course, carbon monoxide and methanol. In the typical acetic acid plant, methanol is imported and carbon monoxide, because of difficulties associated with the transport and storage thereof, is generated in situ, usually by reforming natural gas or another hydrocarbon with steam and/or carbon dioxide. A significant expense for new acetic acid production capacity is the capital cost of the equipment necessary for the carbon monoxide generation. It would be extremely desirable if this capital cost could be largely eliminated or significantly reduced.
Market conditions, from time to time in various localities, can result in relatively low methanol prices (an oversupply) and/or high natural gas prices (a shortage) that can make methanol manufacture unprofitable. Operators of existing methanol manufacturing facilities can be faced with the decision of whether or not to continue the unprofitable manufacture of methanol in the hope that product prices will eventually rebound and/or raw material prices will drop to profitable levels. The present invention addresses a way of modifying an existing unprofitable methanol plant to make it more profitable when methanol prices are low and/or gas prices are high.
Mayland (U.S. U.S. Pat. No. 2,622,089) discloses a method of reforming natural gas to produce hydrogen and carbon monoxide synthesis gas. At column 1, lines 10-36, there is a disclosure that carbon dioxide is combined with natural gas and steam in a reforming reaction to produce hydrogen and carbon monoxide. The specific method is said to obtain larger quantities of hydrogen-carbon monoxide synthesis gas mixtures with a given quantity of carbon dioxide in the feed to the hydrocarbon reforming unit (column 1, line 48 to column 2, line 10). The molar ratio of hydrogen to carbon monoxide in the syngas is said to be 2.0 (for methanol production). The molar ratio of steam to methane in the reformer feed ranges from 1.5 to 1.8, and the ratio of carbon dioxide to methane in the feed ranges from 0.64 to 0.91.
Moe (U.S. Pat. No. 3,859,230) discloses that naphtha and steam are reformed in a two-stage reforming process to make CO and H2. A minor portion (15-30%) of the effluent from the first reformer stage is subjected to absorption/stripping to remove CO2 which is fed to the second reformer stage with the major portion of the effluent from the first stage. The effluent from the second reformer is used as syngas for alcohol/aldehyde production. If desired, the effluent from the second reformer stage can also be treated to remove CO2 that can be recycled to the second reformer stage with the CO2 recovered from the first stage effluent. At column 1, beginning at line 28, it is disclosed that the prior art obtained CO2 from the effluent of the second reformer stage, or from the effluent from the combustion gases used to heat the reformer.
Joensen et al (mentioned above) disclose making acetic acid from CO and H2 by using a dehydration catalyst (Cu/Zn/Al) that produces methanol (MeOH), dimethyl ether (DME) and CO2. The methanol and DME are then separated from the CO2 and reacted with CO to make acetic acid. The CO is said to be made from steam-reformed CH4. A portion of the syngas from the reformer (2 less than H2/CO less than 3) is diverted from the feed to the MeOH/DME reactor and then. membrane or cryogenically treated to recover the CO for feed to the acetic acid reactor. The process is said to be a parallel production of MeOH and CO, avoiding MeOH import as required in prior art processes.
Steinberg et al (U.S. Pat. No. 5,767,165) disclose that CH4 is autothermally decomposed to make carbon black and hydrogen. In FIG. 3, a portion of the CH4 feed is reformed with CO2 (without steam) to make CO. The CO/H2 from the CH4/CO2 reformer is then reacted with the additional H2 from the autothermal reformer to make MeOH. The CO2 is said to be obtained by fossil fuel combustion. Excess H2 from the syngas can be burned to supply energy for the CH4 decomposition.
Park et al (U.S. Pat. No. 5,855,815) disclose making syngas for Fischer-Tropsch synthesis. CO2 and CH4 are reformed with 0-10% O2 and 0-10% H2O in the presence of Ni catalyst containing an alkali metal on a silica support at 600-1000xc2x0 C. and a space velocity of 1000-500,000 hrxe2x80x941 to make CO, H2 and H2O. The effluent is said to have an H2/CO ratio less than 3, compared to an H2/CO ratio in the prior art of 0.5-2 with conventional CO2 reforming.
As far as applicant is aware, there is no disclosure in the prior art for modifying existing methanol plants, including methanol/ammonia plants, to supply stoichiometric MeOH and CO for manufacturing acetic acid, for example, that can be a more valuable product than MeOH.
The present invention involves the discovery that the large capital costs associated with CO generation for a new acetic acid plant can be significantly reduced or largely eliminated by retrofitting an existing methanol or methanol/ammonia plant to make acetic acid. More specifically, carbon dioxide can be fed into a reformer to which natural gas and steam (water) are fed. Syngas is formed in the reformer wherein both the natural gas and the carbon dioxide are reformed to produce syngas with a large proportion of carbon monoxide relative to reforming without added carbon dioxide.
The syngas can be split into a first part and a second part. The first syngas part is converted to methanol in a conventional methanol synthesis loop that is operated at about half of the design capacity of the original plant since less syngas is supplied to it. The second syngas part can be processed to separate out carbon dioxide and carbon monoxide, and the separated carbon dioxide can be fed back into the feed to the reformer to enhance carbon monoxide formation. The separated carbon monoxide can then be reacted with the methanol to produce acetic acid or an acetic acid precursor by a conventional process.
Separated hydrogen can also be reacted with nitrogen, in a conventional manner, to produce ammonia. Also, a portion of acetic acid that is produced can be reacted in a conventional manner with oxygen and ethylene to form vinyl acetate monomer. The nitrogen for the ammonia process (especially for any added ammonia capacity in a retrofit of an original methanol plant comprising an ammonia synthesis loop) and the oxygen for the vinyl acetate monomer process, can be obtained from a conventional air separation unit.
Broadly, the present invention provides, in one aspect, a method for retrofitting an original methanol plant which has at least one steam reformer for converting a hydrocarbon to a syngas stream containing hydrogen and carbon monoxide, a heat recovery section for cooling the syngas stream, a compression unit for compressing the syngas stream, and a methanol synthesis loop for converting at least a portion of the hydrogen and carbon monoxide in the syngas stream to methanol. The method converts the methanol plant into a retrofitted plant for manufacturing a product from carbon monoxide and methanol selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof. The method comprises the steps of: (a) diverting a portion of the syngas stream from at least one reformer to a separation unit; (b) operating the methanol synthesis loop with a feed comprising the remaining syngas stream to produce less methanol than the original methanol plant; (c) operating the separation unit to separate the diverted syngas into at least a carbon monoxide-rich stream and a hydrogen-rich stream, wherein the quantity of hydrogen in the hydrogen-rich stream is greater than any net hydrogen production of the original methanol plant; and (d) reacting the carbon monoxide-rich stream from the separation unit with the methanol from the methanol synthesis loop to form the product, wherein the diversion of the syngas stream is balanced for the approximately stoichiometric production of the methanol from the methanol synthesis loop and the carbon monoxide-rich stream from the separation unit for conversion to the product.
Preferably; at least one steam reformer is modified to increase carbon monoxide production in the syngas stream. The syngas stream preferably comprises carbon dioxide, and the separation unit produces a carbon dioxide-rich stream that is preferably recycled to at least one reformer to increase the carbon monoxide production.
The reaction step can include the direct catalytic reaction of methanol and carbon monoxide to form acetic acid as in the Mosanto-BP process, for example, or alternatively can comprise the intermediate formation of methyl formate and isomerization of the methyl formate to acetic acid, the intermediate reaction of CO and two moles of methyl alcohol to form methyl acetate and hydrolysis of the methyl acetate to acetic acid and methanol, or the carbonylation of the methyl acetate to form acetic anhydride.
In one preferred embodiment of the retrofitting method, the present invention provides a method for retrofitting an original methanol plant that has at least one steam reformer for converting a hydrocarbon/steam feed to a syngas stream containing hydrogen and carbon monoxide; a heat recovery section for cooling the syngas stream, a compression unit for compressing the syngas stream, and a methanol synthesis loop for converting at least a portion of the hydrogen and carbon monoxide in the syngas stream to methanol. The retrofitted plant can manufacture a product from carbon monoxide and methanol selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof. The retrofitting method comprises the steps of: (a) modifying at least one steam reformer for operation with a feed comprising a relatively increased carbon dioxide content; (b) diverting a portion of the syngas stream from at least one steam reformer to a separation unit; (c) operating the methanol synthesis loop with a feed comprising the remaining syngas stream to produce less methanol than the original methanol plant; (d) operating the separation unit to separate the diverted syngas into a carbon dioxide-rich stream, a carbon monoxide-rich stream and a hydrogen-rich stream; (e) recycling the carbon dioxide-rich stream from the separation unit to at least one modified steam reformer to increase the carbon monoxide formation relative to the original methanol plant and increase the molar ratio of carbon monoxide to hydrogen; (f) reacting the carbon monoxide-rich stream from the separation unit with the methanol from the methanol synthesis loop to form the product, wherein the diversion of the syngas stream is balanced for the approximately stoichiometric production of the methanol from the methanol synthesis loop and the carbon monoxide-rich stream from the separation unit for conversion to the product.
The modified steam reformer is preferably modified to operate at a higher temperature to enhance the carbon conversion to carbon monoxide. The separation unit can include a solvent absorber and stripper for carbon dioxide recovery, and a cryogenic distillation unit for carbon monoxide and hydrogen recovery.
The compression unit preferably has a three-stage compressor, and the syngas stream diversion preferably occurs between the second and third compression stages. The third compressor stage is preferably modified for operation at a lower throughput than the original methanol plant. Where the methanol synthesis loop of the original methanol plant includes a recycle loop compressor, the recycle loop compressor can also be modified for operation at a lower throughput.
The method can further comprise the step of reacting the hydrogen in the hydrogen-rich stream with nitrogen to make ammonia. Where the original methanol plant produces a hydrogen-rich stream comprising a loop purge from the methanol synthesis loop that was reacted with nitrogen to make ammonia, the retrofitted plant can use the hydrogen-rich stream from the separation unit as a primary hydrogen source for the ammonia production. With the additional hydrogen available from the syngas, additional ammonia can be produced in the retrofitted plant relative to the original methanol plant.
The method can further comprise installing a vinyl acetate monomer unit for reacting a portion of the acetic acid with ethylene and oxygen to make vinyl acetate monomer. An air separation unit can be installed to make the oxygen for the vinyl acetate monomer unit, and the nitrogen produced from the air separation unit preferably matches the nitrogen required for the additional ammonia production.
In another aspect, the present invention provides a process for making hydrogen and a product selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof, from a hydrocarbon via methanol and carbon monoxide which can be effected by construction of a new plant or retrofit of an existing plant. The process comprises the steps of: (a) reforming the hydrocarbon with steam in the presence of a minor proportion of carbon dioxide to form a syngas containing hydrogen, carbon monoxide, and carbon dioxide having a molar ratio of R ((H2xe2x80x94CO2)/(CO+CO2)) from about 2.0 to about 2.9; (b) recovering heat from the syngas to form a cooled syngas stream; (c) compressing the cooled syngas stream to a separation pressure; (d) diverting a major portion of the compressed syngas to a separation unit; (e) separating the syngas diverted to the separation unit into a carbon-dioxide-rich stream, a carbon monoxide-rich stream and a hydrogen-rich stream; (f recycling the carbon dioxide-rich stream to the reforming step; (g) further compressing the remaining minor portion of the syngas to a methanol synthesis pressure higher than the separation pressure; (h) operating a methanol synthesis loop to convert the hydrogen and carbon monoxide in the further compressed syngas into a methanol stream; and (i) reacting the carbon monoxide-rich stream from the separation unit with the methanol stream from the methanol synthesis loop to make the product, wherein the diversion step is balanced to obtain approximately stoichiometric amounts of carbon monoxide and methanol.
The process preferably has a molar ratio of carbon dioxide to hydrocarbon comprising natural gas in feed to the reforming step from about 0.1 to 0.5 and a ratio of steam to natural gas from about 2 to 6. The methanol synthesis loop can be operated substantially below a total maximum combined design throughput of all methanol synthesis reactor(s) in the loop. The process can further comprise the step of reacting the hydrogen in the hydrogen-rich stream with nitrogen in an ammonia synthesis reactor to make ammonia. The process can also comprise the step of separating air into a nitrogen stream and an oxygen stream and supplying the nitrogen stream to the ammonia synthesis reactor. Where the product comprises acetic acid or an acetic acid precursor which is converted to acetic acid, the process can further comprise the step of supplying the oxygen stream from the air separation unit to a vinyl acetate synthesis reactor, along with a portion of the acetic acid from the carbon monoxide-methanol reaction step, and ethylene, to produce a vinyl acetate monomer stream.
Regardless of whether the plant is a retrofit or a new plant, where the product comprises acetic acid, the reaction step preferably comprises reacting methanol, methyl formate, or a combination thereof in the presence of a reaction mixture comprising carbon monoxide, water, a solvent and a catalyst system comprising at least one halogenated promoter and at least one compound of rhodium, iridium or a combination thereof. The reaction mixture preferably has a water content up to 20 weight percent. Where the reaction step comprises simple carbonylation, the water content in the reaction mixture is more preferably from about 14 to about 15 weight percent. Where the reaction step comprises low-water carbonylation, the water content in the reaction mixture is more preferably from about 2 to about 8 weight percent. Where the reaction step comprises methyl formate isomerization or a combination of isomerization and methanol carbonylation, the reaction mixture more preferably contains a nonzero quantity of water up to 2 weight percent. The reaction step is preferably continuous.